Optical interferometry may be used to make precise measurements in a variety of settings. For example, laser interferometry is currently used to measure small displacements and accurately position stages to nanometer precision in photolithographic processing of semiconductors. As features of semiconductors get smaller, there is a need to achieve even more precise displacement measurements. Using known, mirror based laser interferometry, a portion of the measurement light beam travels in air. If the refractive index of the air in the beam path changes, even locally, the change manifests itself as an apparent displacement. This apparent displacement constitutes a measurement error and the longer the air path, the more serious this error is likely to be. There are a number of known methods to control, reduce or measure changes to the refractive index in the air through which the light travels, but new methods are yielding increasingly marginal improvements.
In addition to measuring displacement magnitude, it is also important that a laser interferometer identify displacement direction. Two known methods for determining displacement direction are the homodyne and heterodyne techniques. The homodyne technique uses a single frequency light beam. The direction of motion is inferred by measuring two or more output signals for each object whose motion is being measured, at least one of which is optically retarded with respect to each other: the phase relationship between these signals indicates the direction of motion. The heterodyne technique uses a dual frequency light source. A reference signal is generated that indicates the phase of the signal formed by mixing the two frequencies directly from the source. For each object whose motion is being measured, a second signal is formed by introducing the light of one frequency into the reference branch, and the light of the other frequency into the measurement branch. Displacement is measured by measuring the phase of a signal formed by mixing these two beams and subtracting the phase of the signal formed directly from the two frequency sources. Any change in this phase difference is related to displacement. A Doppler shift of the measurement beam relative to the reference beam indicates the amount and direction of velocity. The heterodyne technique permits the direction of motion to be identified using a single detector and has enhanced immunity to low frequency noise relative to the homodyne technique. Thus, the homodyne scheme uses a simpler source but requires at least two detection channels per measurement axis that must be matched in gain and phase. The heterodyne scheme uses a more complex source, but requires only a single detector for each measurement axis plus a single additional detector for the laser source.
Encoders to measure displacement are also known. Because encoders measure displacement that is transverse to the measurement beam, encoder technology can be used to minimize the need for long air paths. Typically, encoders use the homodyne technique. As an example, a device made by Heidenhain uses a system of three detectors in order to determine the direction of motion. Unfortunately, it is difficult to sufficiently match the gains and phases of the detectors and their associated electronics to allow measurement with nanometer or sub-nanometer precision. This difficulty is exacerbated if the measurement signals travel along cables which flex or move. Accordingly, encoder measurement displacement systems are used for applications that require lower precision than what is currently available with laser interferometry displacement measurement systems. As in the case of interferometers, homodyne encoders are susceptible to low frequency noise.
There remains a need for an improved method and apparatus for measuring and controlling displacement with higher resolution than previously available under the prior art.